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Curious connections The gene responsible for the loss of our ability to feel pain is also involved in the loss of our sense of smell, an international team of researchers have found.

Professor Frank Zufall, of the University of Saarland School of Medicine, in Germany, and colleagues, report their findings online this week in the journal Nature.

The researchers tested three people in their 30s with a rare genetic inability to feel pain (congenital analgesia) and found they were unable to smell at all (a condition known as anosmia).

Interestingly, none of the subjects had been aware that they could not smell.

Being unable to feel pain sounds enticing, but people with congenital analgesia frequently bite their tongues, break bones or burn themselves without being aware of it, sometimes leading to severe damage.

On the plus side, two of the individuals in the study had given birth with completely pain-free labour.

It is known that the inability to feel pain is due to a particular defective gene (SCN9A), which codes for a particular type of sodium channel protein. These sodium channels are essential for pain nerves to be able to send messages.

The researchers wondered if the sodium channels could be important in smell detection too.

The team first showed that the olfactory sensory nerves that relay smell information in both humans and mice did indeed contain the sodium channels.

Then they produced a genetically-altered strain of mice that lacked the sodium channels and were unable to smell.

They compared these mice with normal mice, carefully recording the electrical activity of single nerve cells as the animals were exposed to smells.

Surprisingly, the nerve cells responded normally to smell, but the signals were not reaching further into the brain.

The sodium channels appear to be essential for triggering the release of neurotransmitter, which is essential for transmission of information from one nerve cell to the next.

Zufall sees this work as being of fundamental importance in understanding how the brain processes smell, but there could be other advantages.

"You can imagine a spray which you use to temporarily knock out your sense of smell", he suggests, "which may be useful for people working in situations with awful smells".

Smell (or lack of it) may be a side-effect we have to consider when we take a drug for severe pain in years to come, as several pharmaceutical companies are racing to develop pain relief drugs targeting these particular sodium channels.